ComboBox is an ASP.NET AJAX control that, like the AutoCompleteExtender, combines the flexibility of a TextBox
with a list of options that users are able to choose from. It borrows many of its
properties, behaviors, and naming conventions from the Windows Forms ComboBox control, and is derived from the
same base class as the ListBox, BulletedList, and DropDownList web controls. In
fact, a ComboBox is best described as a DropDownList that can be typed directly
into like a TextBox.

Working with a ComboBox in code is also a lot like working with a DropDownList.
It has all of the same properties and events as a DropDownList, with a few additional
ComboBox-specific properties and events. Firstly, it can be configured to either
prevent or allow user-typed text that does not match an item in the list. When user-typed
text does match an item in the list, the ComboBox can also be configured to auto-complete
the text based on the first matched item in the list, to show the list and highlight
the first matched item, or to do both simultaneously. When user-typed text does
not match an item in the list, the ComboBox raises ItemInserting and ItemInserted
events which can be handled during postback. Other than these special behaviors,
the ComboBox behaves essentially like a DropDownList.

The ComboBox is intended as a supplement to, rather than a replacement for, the
AutoCompleteExtender. Though there are many scenarios where either could be used
to satisfy the same user interface requirements, there are scenarios where one control
could offer particular advantages or disadvantages compared to the other:

Data Binding - The ComboBox can bind to data source controls like SqlDataSource
and ObjectDataSource, and to runtime objects that implement IListSource, IEnumerable,
or IDataSource. Like the DropDownList, the ComboBox also has an Items collection
that can be manipulated declaratively and/or programmatically. The AutoCompleteExtender
can only get its item list from a web service or page method. When it is possible
and more convenient to bind to a data source control, runtime object, or declared
item list, the ComboBox may be a better choice than the AutoCompleteExtender.

Restricting User-Typed Text - Another feature of the ComboBox is that it
can restrict input so that an item in the list is always selected after a postback
(as long as the Items collection is not empty). The AutoCompleteExtender allows
users to type anything into the TextBox being extended, even if the typed text doesn't
match any options returned by the extender's ServiceMethod. A ComboBox may be
a better fit for user interfaces which require a predetermined item be selected
from the list (like a foreign key input).

Composite Items - Items in a ComboBox, like items in a DropDownList, have
both Text and Value properties. The only user input value offered by the AutoCompleteExtender
is the Text property of the TextBox being extended. If the items in your list can
be modeled with different Text and Value properties, the ComboBox may provide a
better match for the data items being listed (again, foreign keys can be a good
example of this).

Long Item Lists / Multiple Input Controls - All of the items in a ComboBox's
list will be rendered to the web page it exists in. The AutoCompleteExtender, on
the other hand, retrieves items from its ServiceMethod after the page is rendered.
When your ComboBox contains a rather long list of items, or when you have a relatively
large number of ComboBoxes on the same page (or within the same UpdatePanel), load
times could be slowed down significantly. When ComboBoxes perform slowly because
of the amount of markup they must render to the browser, an AutoCompleteExtender
can be used instead to increase performance.

Partial Auto-Completion - The auto-complete feature of the ComboBox will
only match items that start with the first user-typed character. An AutoCompleteExtender's
ServiceMethod, on the other hand, can be configured to match items where the user-typed
text lies somewhere after the first character in the item. A ComboBox cannot be
used in application scenarios that require items to be "partially matched"
like this.

Multiple Item Selection - The ComboBox, like the DropDownList, will only
allow one item to be selected at a time. The AutoCompleteExtender can be configured
to allow users to select multiple items simultaneously (using the AutoCompleteExtender's
DelimiterCharacters property), like a ListBox or CheckBoxList. When users must have
the ability to submit multiple items in a single postback, the AutoCompleteExtender
should be used instead of the ComboBox.

ComboBox Properties

The control above is initialized with this code. The italic properties
are optional:

DropDownStyle - Determines whether the user is allowed
to enter text that does not match an item in the list, and whether the list is always
displayed. If "DropDownList" is specified, users are not allowed to enter
text that does not match an item in the list. When "DropDown" (the default
value) is specified, any text is allowed. If "Simple" is specified, any
text is allowed and the list is always displayed regardless of the AutoCompleteMode
property value.

AutoCompleteMode - Determines how the ComboBox automatically
completes the text that is typed into it. When "Suggest" is specified, the ComboBox
will show the list, highlight the first matched item, and if necessary, scroll the
list to show the highlighted item. If "Append" is specified, the ComboBox will append
the remainder of the first matched item to the user-typed text and highlight the
appended text. When "SuggestAppend" is specified, both of the above behaviors are
applied. If "None" (the default value) is specified, the ComboBox's auto-complete
behaviors are disabled.

CaseSensitive - Specifies whether user-typed text is matched
to items in the list in a case-sensitive manner. The default is "false".

RenderMode - Specifies whether the ComboBox is rendered
as an "Inline" or "Block" level HTML element. The default is
"Inline".

ItemInsertLocation - Determines whether to "Append"
or "Prepend" new items when they are inserted into the list, or whether
to insert them in an "Ordinal" manner (alphabetically) based on the item
Text or Value. The default is "Append".

ListItemHoverCssClass - When specified, replaces the default
styles applied to highlighted items in the list with a custom css class.

ListItem - One or more child controls used to declare
items that will be added to the ComboBox list. When bound to a data source, all
declared ListItems will be removed unless the AppendDataBoundItems property is set
to "true".

ComboBox Known Issues

When ListItemHoverCssClass is specified and the ComboBox list is scrollable, highlighting
a list item will cause the scrollbar to flicker when using the Internet Explorer
web browser. To avoid this issue, do not specify the ListItemHoverCssClass property.

ComboBox Theming

You can change the look and feel of the ComboBox using the ComboBox CssClass property.
The ComboBox has a predefined set of CSS classes that can be overridden. It has
a default style which is embedded as a WebResource and is a part of the Toolkit
assembly that has styles set for all the sub-classes. You can find the default styles
in the Toolkit solution in the "AjaxControlToolkit\ComboBox\ComboBox.css"
file. If your CssClass does not provide values for any of those then it falls back
to the default value. In the example above the default style is displayed when "[Empty
String]" is selected as the CssClass. To customize the same the user would
have to set the CssClass property to the name of the CSS style and define the styles
for the individual classes so that the various elements in a ComboBox control can
be styled accordingly. For example, if the CssClass property was set to "CustomComboBoxStyle",
this is how the css to style the border and background color would look: